Before around 2.5 or 2 billion years ago, atmospheric oxygen levels were very low. Then, around 2 billion years ago there was a sudden boom in the amount of oxygen (O2) in the atmosphere.

http://www.ecologicalpsychology.com/perception1.html

CO2 in the atmosphere is dropping at the same time. At least some of this atmospheric carbon was probably becoming incorporated in the biosphere.

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